A direct drill bit drive can comprise a respective drive section, gear section and bearing section in which all bearings for the axial support, part of the bearings in the drive section, and all bearings in the drive section, and all bearings in the remaining sections for radial support, together with the gearing, are disposed in an oil-tight closed annular space between a non-rotary outer tube and several rotary inner tubes which are disposed in series in an axial direction, this annular space also acting as an oil reservoir, and the flushing medium being fed through the concentrically disposed inner tubes to the drill bit, which is fixed to the inner tube located in the region of the bearing section.
Alternatively a direct drill bit drive can comprise a respective drive section and bearing section, in which all the bearings for axial support, part of the bearings in the drive section, and all the bearings in the remaining sections for radial support are disposed in an oil-tight closed annular space between a non-rotating outer tube and several rotating inner tubes which are disposed in series in an axial direction, this annular space also comprising an oil reservoir, and the flushing medium being fed through the concentrically disposed inner tubes to the drill bit, which is fixed to the inner tube located in the region of the bearing section.
Such drives are subjected to very severe axial and radial stress. Very strong drilling impacts--the severity of which depend on the drilling method and in particular on the type of rock being drilled--act on the drilling shaft, the gearing and on the drill drive, for example a turbine, so that such installations are not always operationally reliable because of the high impact stress, and in particular can work for only a short time before overhaul, and thus have a short life. The service life of the drill bit cannot at present be utilized fully because of damage to the bearing unit, the gearing or the drive itself.
The publication "Artep Institut Francais du Petrole" discloses a direct drill bit drive comprising a reduction gear unit in accordance with the first approach described afore, in which the entire drive is made axially rigid, and is thus prone to the aforesaid drawbacks. Such drawbacks also arise in the case of other known drill bit drives without gearing, that is, drives in which the rotational speed of the drive section and the required rotational speed of the drill bit are equal.